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New NIOSH Sound Level Meter App

Image of the main screen of the NIOSH SLM app (shown with a MicW i436 external microphone)

Imagine if workers around the world could collect and share workplace (or task-based) noise exposure data using their smartphones. Scientists and occupational safety and health professionals could rely on such shared data to build job exposure databases and promote better hearing health and prevention efforts. In addition, the ability to acquire and display real-time noise exposure data could raises workers’ awareness about their work environment and help them make informed decisions about potential hazards to their hearing.

The idea was so intriguing that in 2014, the NIOSH hearing loss team evaluated 192 sound measurement applications (apps) for the iOS and Android platforms to examine their suitability and accuracy in relation to professional sound measurement instruments (Kardous and Shaw, 2014). Of the 192 apps the team examined, 10 iOS apps met the outlined criteria for functionality, features, and calibration capability, and of those, 4 iOS apps met our testing criteria. Read more about that study in the blog So How Accurate Are These Smartphone Sound Measurement Apps?

Realizing that most of the apps on the market are oriented at the casual user and lack the accuracy and functionality necessary to conduct occupational noise measurements, NIOSH hearing loss researchers collaborated with an app developer, EA LAB, to create an iOS based sound level meter app that measures and characterizes occupational noise exposure similar to professional instruments.

The NIOSH Sound Level Meter (NIOSH SLM) app for iOS devices is now available on iTunes freely to the occupational safety and health community as well as the general public.

The app was subjected to the same testing requirements that were established in the NIOSH laboratory study.[1] It met the testing criteria (± 2 dB mean difference from the reference type 1 sound level meter). In our most recent study on the accuracy of apps when used with external calibrated microphones, the 4 apps from our original study achieved closer agreement (within ± 1 dB) of the reference type 1 sound level meter (Kardous and Shaw 2016). The NIOSH SLM app, when used with an external calibrated microphone, measured sound levels within ± 1 dB of the reference SLM over the testing range of 65 -95 dB SPL in our laboratory. While the app is not meant to replace a professional sound level meter or a noise dosimeter or be used for compliance purposes, we recommend that those interested in making proper noise measurements use an external microphone that can be calibrated with an acoustical calibrator for improved accuracy (Roberts et al. 2016).

The NIOSH SLM app has many important features, it provides a readout of the sound level using the built-in microphone (or external microphone if used) and reports the instantaneous sound level in A, C, or Z-weighted decibels. View the video for a demonstration of the app’s features. The weighting is user-selectable and can be accessed in the “Settings” screen. The app also reports the main metrics that are of importance for proper occupational noise measurements – mainly the run time (total time), the A-weighted Equivalent Sound Level (LAeq), the Maximum Level measured during the current run time, the C-weighted Peak Sound Pressure Level (LCpeak), the Time-Weighted Average (TWA) and Dose. The app also contains some basic information on noise and hearing loss prevention. In addition, the app allows the user to save and share measurement data using the smartphone other communication and media features. If location services are enabled, the app can utilize the GPS feature to provide an exact geospatial location of the location of the noise measurement.

Your input on the new app is appreciated as we try to improve it and make it widely accessible. Help us spread the word about this new tool for protecting workers’ hearing.

June 2018 Update: EA LAB and NIOSH researchers have evaluated the app’s performance as part of a system (iPhone + external microphone) for compliance with type 2 requirements of IEC 61672/ANSI S1.4 standard: Sound Level Meters – Part 3: Periodic Tests. The results were published in the Applied Acoustics Journal [Celestina et al. 2018].

[1] Please note that professional sound level meters must comply with a host of acoustical and electrical tests to meet national and international standards. As of today, no smartphone or smartphone-based app has met the requirement of such standards. Although we tested and verified the accuracy and functionality of this app at the NIOSH Acoustics Laboratory (over a specific testing range), this app does not comply with any national standard. We are currently conducting a study to evaluate the app’s performance in various workplace settings. In addition, the app was not designed to calculate noise exposure metrics based on environmental or non-occupational noise limits.

111 comments on “New NIOSH Sound Level Meter App”

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Thank you for the question, Asmaa. We discuss this issue on the frequently asked question section of our page https://www.cdc.gov/niosh/topics/noise/app.html. Basically, due to the fact that the Android marketplace is fragmented among many manufacturers with each having different hardware and software tools, it made it nearly impossible to verify the performance and accuracy of such an app on every model on the market today. If you’d like more information, see one of our first publications listed in the references section.

In my country the external microphone is not available at the same time i am trying to use the app with inbuilt microphone. Does it make much difference or just a few plus and minus with external and inbuilt microphone?

The main advantage to using an external microphone is the ability to calibrate the microphone before each measurement, not that it is more accurate than the internal microphone. The internal microphone are very capable and have good frequency response and dynamic range but they are hard to calibrate. The app comes with a nominal sensitivity for the internal microphone that allows the measurements to be within ± 2 dB(A).

This article states “Of the 192 apps the team examined, only 4 iOS apps met the outlined criteria for functionality, features, and calibration capability”. Unless I am misreading your cited source, the number of iOS apps that met the criteria should be 10, with 4 being the number of the Android apps partially meeting the criteria. Please correct me if I’m wrong though.

Thank you for the comment, Andrew. You are correct, we looked at 192 iOS and Android apps, we selected 10 iOS apps, and of those 10, only 4 iOS apps met our testing criteria. We modified the second paragraph above to improve clarity.

Please see our study on external microphones http://asa.scitation.org/doi/full/10.1121/1.4964639, we examined the MicW i436 and the Dayton Audio iMM-6. Both microphones performed well in our study, though each has its pros and cons which we detail in the study. We have also evaluated the Etudige EIM-003 for our study, though that microphone does not fit the typical ¼” adapter for acoustical calibrators. Please note that references to products and services do not constitute an endorsement by NIOSH or the U.S. government.

Love the app and I am using it on an IPAD. However, I noticed when I reset the calibration or changed it to OSHA, it said “measure must be stopped before changes…” After I make the adjustment, the app stops taking measurements. How do you restart it? I close the app and go back and still not measurements are recorded.

The only way to get it to work is when I uninstall and re-install the app.

app loaded fine and worked well. then stopped working. deleted and reloaded the app several times. on the main reading screen (Instantaneous level (dBA)) I have red text that says “overload.” Now what?

A new update of the app will be released tomorrow and will address this issue. Sometimes there is a conflict when another app is trying to use the microphone. In the meantime, we suggest closing the app completely (press home key twice then swipe the app screen upward to close it) and then try again.

You might expect a manufacturer of noise measuring instrumentation to launch into a negative, stereotypical tirade but at Casella we understand the ubiquitous nature of the phone app and we fully support anything that generates more awareness of noise in the workplace. Two comments however. Firstly I would recommend the use of an acoustic calibrator such as our CEL-120/2 with a 1/4″ microphone adaptor . Secondly had you considered resurrecting the Type 3 Indicator grade of instrument classification that existed in ASNSI S1.4-1971 (and BS EN 60651) back in the day? Since current US & EU noise legislation would not recognise the results of a type 3 instrument, it would both legitimise and caveat their use.

Thank you for your comment. We agree regarding the use of acoustical calibrators, at the expense of repeating ourselves, we wrote above “we recommend that those interested in making proper noise measurements use an external microphone that can be calibrated with an acoustical calibrator for improved accuracy.”

The issue regarding resurrecting ANSI S1.4-1971 would be up to ANSI.

We remind readers that references to products and services do not constitute an endorsement by NIOSH or the U.S. government.

With regards to android, couldn’t you just release it and ask folks to either use a calibrated external microphone or to calibrate their device’s mic?

Also, have you seen audiotool? It was around when you guys did the 1st study, on both Android and iOS, and looks like one of the more serious, full featured apps available. Puzzled why it wasn’t included.

Thank you for the comment, it’s not as simple as releasing an app on Android and then instructing people to use it with an external mics – some people may not go through the NIOSH links or see our instructions. To release this app, we had to test it on all available iOS devices and ensure that they all perform according to our criteria, with the built-in microphone and with external microphones.

As for your other question, If you are referring to the AudioTools app, we did include that app in our studies. If you are referring to AudioTool, that app did not meet our selection criteria for occupational type of noise measurements.

I downloaded the app and let it run for a few minutes in the office. Worked fine. The next time I tried the app It did not seem to work and shows in red letters “overload”. What is going on and how can I restore operability?

Thank you for the input, David. You can share the noise report, in pdf format, via email. If you like to attach a picture of the screen, then I would take a screenshot (using the home and ON button) and then in the email screen that has the pdf report, I would add the picture as an attachment. It’s a great suggestion, we may consider making this process simpler in upcoming updates!

Excellent app, I tested it in the worked areas with different nivels of noise, using an IPhone 7 plus, and comparing the results with a professional equipment; the results was very close (+/- 1dB compared with the professional equipment). I found a little bit more difference in closed areas than in open areas.
Nice app!!

Great idea and achievement. However, the fact that it is only for iPhones limits its use. Also, it sounds like recommending the iPhone. I know that this is not your intention at all, but still. Please review your decision to exclude the Androids!

Basically, there are many Android manufacturers and each manufacturer has their own set of specs and different parts and chips that they use, not to mention the many different Android OS’s out there. In addition, some Android makers use their own audio processing, some use third party, which can introduce latency in some devices. On the other hand, Apple devices have a common software architecture called Core Audio that makes dealing with audio signals uniform across all its devices. In our studies (see the blog references above), we would find the same app provide different readings on different devices and this could also explain why the sound measurement apps market on Androids is much less developed as far as features and performance than you can find on iOS. To release the app on Android devices, we would basically have to test and verify the performance of the app on every Android device out there, all with different combination of Android OS’s as well, an impossible task at this time. We are exploring several options at the moment since we recognize the importance of the Android market.

If you, or any of our blog readers, have any suggestions or ideas for dealing with this issue, we definitely welcome them.

First I want to thank Captain Kardous and his colleagues for the studies and hard work that lead to the development of this app.

@Alberto Behar
As explained by the Captain, recording apps for Apple devices are designed around a very well build low-level API (Core Audio). Apple gives the app developers access to this low-level API through a framework (Audio Toolbox or Audio Unit). This architecture makes it easier to deal with raw PCM data (audio signals) recordings uniformly across all the devices as Core Audio isn’t affected by the hardware. Microphones and other electronic components used in the iDevice is what can change. However, Apple is doing a good job at choosing good quality parts and variations between iDevices (for the kind of measurement we are looking for (peak values of PCM data)) can be corrected with proper calibration. Apple has much less SKUs than the entire Android market, making it easier to find the variations in the lab and correct them.

Also, Android devices are subject to more latency in the audio signal processing. The main reason for that is that the low-level API (OpenSL ES) for audio recording is not very well integrated across all the operating system versions. Also, OpenSL ES is missing some features that requires some hard work in C and C++ language to code the desired calculations. Developers are using public JAVA APIs instead (AudioRecord or MediaRecorder) that has huge audio latency and might not be suitable for real-time measurements.
As for the hardware part of the issue, there are more than ten popular Android phone brands on the market, all using various components.

All things considered, it is indeed an impossible task to test and verify the performance of the app on every Android device on the market to find a proper calibration.

I have asperger and workshop in an Office environment. I would like to use this tool to measure the total sound dosis received every day. For concentrated work the advised level is about 45db or less. That is a much lower than the level for hearing damage. Is it possible to add these lower levels to the app so we can use it also for office sound level dose? Is there a way that I calculate the dose myself for this lower level based on 80 db? I’m also thinking about sharing the data between colleagues. Would be handy if the data could be exchanged in binary format with some kind of service for reporting on a larger scale. Think about the best office to work for contest.

Thanks for your comment, Tauvic. The app was designed to measure noise exposure in the workplace and is aimed at industrial hygienists or occupational safety and health specialists. The 45 dB is a non-occupational limit for office environments and indoor spaces and thus it is beyond the scope of our mandate at NIOSH. There are instruments out there that allow you to set the threshold at 40 dB (or no threshold at all), there is no way to calculate the dose otherwise, unless you find a way to record the actual sound waveform in your office and do the calculations based on that recording of the raw signal.

Thank you for the suggestion for using binary data, this is something that we might consider in future updates.

I have noticed this as well. I also get higher readings with my iPad to a degree of about 10 decibels. I don’t use external mice but have ordered one to see if there is a difference as well. Which one should I use?
Thanks.

Some users have reported this ‘overload’ problem and it’s due to the interface not refreshing. We’re aware of this bug and we’re working on a fix, expect a new update soon. In the meantime, please try to turn off your iPhone and let it reboot and it should have resolved it.

I haven’t downloaded and tried the app yet, but it seems intended mainly for handheld use with a user in constant attendance.

Some kinds of noise exposure happen infrequently or unpredictably. For example, neighborhoods near me have started experiencing increased noise from overflights connected with a large airport within five miles, as air traffic routings and protocols are modified to exploit the potential of GPS. Noisy overflights happen unpredictably, so it would be useful to have the measuring device in standby mode, tracking noise levels, and springing into more intensive action when a threshold is exceeded.

Beyond measuring the levels for a single weighting, it would be more useful if enough information could be retained so the levels could be calculated and displayed under multiple weighting schemes and different metrics, e.g. instantaneous maximum, L10, L90 and so forth. It would also be good to save recordings of noise incidents, perhaps including some seconds before and after the actual threshold-exceeding period.

The overflight example is only one scenario. Others that have come up in my neighborhood have been late-night street racing, construction noise, loud custom mufflers and freeway noise. A smartphone-based application for extended monitoring would be extremely helpful in all these instances and surely many others.

Hello Dave and thank you for the comment, you bring up some interesting points.

It’s true, the app is intended for the user to be in constant attendance. Our aim is to help the occupational safety and health specialist make an easy and quick noise measurement in the workplace – the app measures and calculates the relevant occupational noise exposure metrics and reports noise levels and averages in A, C, or Z weighed decibels. The user can also save measurements, generate reports, or upload/share those measurements.

As for your suggestion to have the app running in the background constantly and then records events when certain thresholds are exceeded, while possible, it would most likely require a dedicated smartphone – no phone calls or running any apps that need access to the microphone, very careful handling (can’t be taken in and out of pockets and such), and most importantly, different features since our app is intended to measure occupational noise instead of environmental noise. I believe there are some studies that have used smartphones as dedicated sound measurement tools. Some researchers have explored the use of older, refurbished smartphones with a dedicated app, to build a cheap noise monitoring network, mostly in urban settings. I believe limitations regarding battery life, proper calibration, and reliability continue to hinder their wide adoption for more than conducting research studies though.

Thank you for the kind comment, Dr. Deshpande. There is no current plan to display the frequency spectrum though we may consider it if there’s enough interest. There are some apps out there that offer frequency spectrum, check our earlier study on smartphone apps http://asa.scitation.org/doi/full/10.1121/1.4865269, several of the apps we tested offer various displays. For instance, NoiSee from EA LAB, our partner, offers octave band display. SPLnFFT (Paid app) offers Octave and FFT displays.

Some users have reported this ‘overload’ problem and it’s due to the interface not refreshing. We’re aware of this bug and we’re working on a fix, expect a new update soon. In the meantime, please try to turn off your iPhone and let it reboot and it should have resolved it.

Good to see the NIOSH app. I compared it using the SoundLog app on an iPhone 5S and the standard internal microphone. The NIOSH app seemed to be consistently low by about 8dB compared to an inexpensive sound level meter, a UNI-T UT353.The SoundLog results were within +/- 1 dB of the UNI-T.
Sound levels were provided by a 1000Hz tone produced by a loudspeaker.

The SoundLog is an excellent app from the National Acoustic Laboratories, an Australian government research institution that conducts research on noise and hearing loss, similar to the NIOSH hearing loss prevention program. SoundLog displays the LAeq (A-weighted, equivalent sound level), LCpk (C-weighted peak sound level) and LAeq.8h (A-weighted, equivalent sound level averaged over 8 hours). Our app displays the instantaneous sound pressure level as a default (in A, C, or Z weighting), but you can select to display the LAeq, Max Level, LCpeak, TWA, and Dose. So when you do comparisons, I suggest comparing the LAeq on both apps, I just ran a quick test (pink noise through speakers) and the LAeq on both apps are with 1 dBA of each other’s and with the Larson-Davis model 831 Type 1 sound level meter.

We are exploring several ideas regarding Androids. See my response above to Alberto Behar on February 6th as to the reasons why the app is not available on Android devices. We also provide a brief explanation in the FAQ section here: https://www.cdc.gov/niosh/topics/noise/app.html

We have tried to see if we can get different readings on our iPads and iPhones in our lab to measure various noises, but have not seen similar results. Only at very low ambient background levels, there’s some discrepancy between the iPad and the iPhone readout but that can be explained by the different sensitivity of the microphones. Are you running the same OS on both devices? Also, the iPads have a back-facing noise-cancelling microphone, in addition to the front-facing one that may be causing some issues for some users. Try to go into Settings, General, Accessibility and see if the Noise Cancellation is enabled or disabled on both devices?

Thanks for sharing this post. Android’s source code is released by Google under an open source license, although most Android devices ultimately ship with a combination of free and open source and proprietary software, including proprietary software required for accessing Google services. Android is popular with technology companies that require a ready-made, low-cost and customization operating system for high-tech devices.

Hello Emily and thanks for your interest. In our study, we evaluated two microphones, the MicW i436 and the Dayton Audio iMM6. Both microphones performed well over the 65-95 dB (pink noise) testing range in our lab, when calibrated. We used the Larson-Davis CAL200 acoustical calibrator in our study.

Thank you, Max. Measuring peak impulse sound levels, especially from gunfire, require some dedicated and specialized equipment. We’ve done some studies in the past where all of our noise dosimeters clipped above 143 dB SPL. Gunfire can measure anywhere from 160-170 dB, and the highest level we were able to capture with the app is 139.2 dB SPL. In other words, this app – or any smartphone app for that matter – cannot capture peak sound levels accurately.

Hi Martin, thanks for bringing that specific microphone to our attention. Not having tested that specific mic, I can’t offer any empirical judgment. However, I would tend to think it will work as well as the mics we tested, it has excellent specifications and the manufacturer claims it is IEC 61672 class 2 compliant. The main point with any of these external microphones, is the ability to calibrate them using commercial acoustical calibrators and available adapters (a 1/4” adapter would work perfectly with this mic). It doesn’t look like it offers a lightning port adapter so will not work with iPhone 7 and later.

In your response to Martin you state “It doesn’t look like it offers a lightning port adapter so will not work with iPhone 7 and later.” Is that true? iPhone 7, 8 and X all come with an adapter for “lightening port-to-audio jack.” Wouldn’t that solve the problem and make it work for the later iPhone models?

Great question, Frank. Yes, it is true that newer Apple products do come with a lightning port to 3.5 mm headphone jack adapter. The issue with that arrangement is that it will leave the microphone ‘dangling’ from the adapter. To make a good noise measurement, the orientation of the microphone is important and a dangling cable (and microphone) may present some issues to how the user handles and orients the microphone. Some microphone manufacturer make microphones that can plug directly into the lightning port (e.g., MicW’s i437L and Studio Six Digital’s iTestMic2). The microphone mentioned above by Martin, does not seem to offer a lightning port adapter that can be directly plugged into newer Apple devices.

Love the app, but I found it only after doing an online search for sound meter apps. Your app would likely find a broader audience if the AppStore listing made it more findable. For example, consider calling it “NIOSH SLM Sound Level Meter.” Cheers!

Thank you for the kind endorsement of the app, Steve. We have received similar comments in the last several months from users having a hard time finding the app, and now with the introduction of iTunes 12.7, Apple has removed access to the App Store from its desktop versions, so that makes it even harder for users to find our app without using specific search terms. We are still learning how App Store users search and find apps so this suggestion and feedback is very valuable to us and we will consider it in the new update we’re currently working on!

Will the app recognize microphones plugged in directly to the Lighning port like the MicW i437L mentioned above? We had tested a consumer Sennheiser microphone with a lightning connector on an iPhone8 about 4 months back, and it wasn’t recognized at all.

Yes, the app will recognize an external microphone automatically, we have tested it with the MicW i437L and SSD iTestMic2. Please let us know if you still encounter any issues, and the model of the Sennheiser microphone you use.

Dear Captain Kardous,
Could you please speak to changes in the newest update, specifically regarding the “dose?” While in the immediately previous version, “dose” was a running tally, and “projected dose” an extrapolation of that measure over 8 hours, the basic “dose”measure now appears to be the latter. For example, in the older version, if I whistled directly in to the mic, dose would slowly rise 1, 2, 3, etc. %. Now doing so spikes the dose well over 100%. This value drops the longer the recording continues, as quieter readings are averaged in.

The documentation does not seem to address this change (although in the help page of the new version there is only one landing page and no further info?).

Dear Chuck, I’m just starting to use your App for noise monitoring at home. I know the App is set for industrial purposes but I think it suits my purpose. Our local Government sets the recommended noise level for residential areas at LAeg, adj., 1hr. I’m not sure if that’s equivalent to the LAeq set in the NIOSH App. Could you advise?

Dear Greg, so it seems that your local government is setting LAeq, the A-weighted, equivalent sound level (average level) measured over 1-hour period. If you plan to use the NIOSH app, my suggestion is to run it for exactly one hour and report the LAeq value (third row). You can also press the LAeq and it will appear at the top for easier reading. Make sure you save the measurement(s) using the save button at the bottom after each 1-hour measurement so you have those records.

Thank you for your suggestion, Suzi. We’re always looking for ways to improve users’ experience so we will take your suggestion into account for a future update. Right now, you can save the measurement with a timestamp (date and time, along with a measurement duration), and share it in .pdf and .html formats. Saving data points may require additional permissions and availability of adequate storage on your device.

While the app recognizes the MicW and itestmic2 when attached thru the lightening port on new iphones, have you tested whether measurements are the same as having the mics on the headphone port, assuming that the position of the mic on the cable is stable and oriented to the sound source ( re your earlier comment)

Yes, we have tested the NIOSH SLM app with new external microphones that connect directly to the lightning port, including the MicW i437L and SSD’s iTestMic 2. The MicW microphones work very well out of the box (within 0.3-0.5 dB SPL when calibrating using an acoustical calibrator). The SSD iTestMic2 did not perform as we expected with our app, or another reputable SLM app.

Dear Chuck,
is the NIOSH SLM app suitable also for measuring low frequency noise, i.e. 100 Hz ? The large power transformers in electrical substations in Europe emit mainly at this frequency and harmonics. Maintenance workers are exposed to these low frequency components. Thank you very much.

Hello Roberto, the MEMS microphones used in smartphones (or if you use an external microphone) can measure noise levels as low as 20 Hz according to their manufacturers. For low frequency measurements, you will need to choose Z-weighting from the settings instead of the default A-weighting. However, for such types of measurements, it is best to use a professional sound level meter, with an appropriate microphone, that can provide 1/3 octave band measurement features, so you can see/display the exact contribution from such sources.

Thank you, Chuck.
Only a short reply.
Our goal is to test the suitability of apps for a very preliminary survey of noise around transformers, that can be done directly by the workers using smartphones.
If MEMS microphones are good in measuring until 20 Hz, is there some issue related with SW and with the working out of the acquired signals ? Why it is best to use a professional SLM ? Thank you.

Hi Roberto, there shouldn’t be an issue with the software of the app, all it’s doing is taking the electrical signal from the microphone, digitizing it, and applying some calculations to convert those signals into decibels. The microphone’s suitability for low-frequency measurements is the most critical component here. For a preliminary assessment, the app should be fine. You should plan to make two sets of measurements, one with the default A-weighting and one with the Z-weighting selected. You can then compare the two measurements to understand the low-frequency noise contribution from the transformers. The Z-weighted noise levels will higher, depending on how much of the noise is low-frequency. There are apps that offer octave and 1/3-octave band analysis, see the apps from our study on smartphone apps SoundMeter, SPLnFFT, NoiSee, and SPL Pro all offer some sort of frequency or octave-band analysis, at some nominal cost.

A measurement with a professional SLM is suggested if you are trying to conduct a full assessment and isolate the noise emitted from the transformers, especially if you plan to implement some engineering controls to mitigate any harmful exposures to the workers.

Thank you for the valuable feedback, Husein. We are gradually trying to increase our exposure on the AppStore. We had a similar suggestion few months ago and we made some changes in the naming and keywords used, and added an option to allow the users to rate the app and that appears to have helped a bit. However, if you, or any of our valuable readers, have other suggestions for making the app more findable, we would certainly appreciate it.

Hi John, it’s not likely that exposure to 72 dB is going to cause hearing damage. Our recommended exposure limit is 85 dB (A-weighted), and that’s over 8-hours a day, 40 hours week, and years of exposure. We do recommend hearing protection for sound levels above 85 dBA. Questions I would ask, how far was your iPhone from the amplifier speaker, was it pointed (bottom side where the microphone is) at the speaker, I would also try to take an average, you can do that by selecting LAeq and let it run for 10-15 seconds. It could be that the measurement is too low.

Having said all that, muffling in the ear is a symptom that often needs medical attention, especially if it happens repeatedly after you play your guitar. We do recommend that you follow up with a health professional, your physician or an audiologist.

I downloaded the Sound Level meter app on an iphone 5s and and an iphone 8. When starting the app, both phones do not display a sound level and both say “Overload”. The microphones do work in other apps.
Is there a correction for this?

Hello Tom, Please make sure that you allow the app the permission to use the internal microphone, a message pops up the first time you open the app (“NIOSH SLM would like to access the microphone”), please press OK to give it permission to use the internal microphone.

Hi, we can’t comment or endorse any specific product, we use Type 1 and Type 2 acoustical calibrators from the main sound instrument manufacturers such as Larson-Davis, 3M Quest, and B&K among others. Those types of professional instrument calibrators are priced in the several hundred to over a thousand price range. There are cheaper options on the market but we cannot vouch for their accuracy or reliability, though they should be adequate for general purpose, Type 2 level of accuracy, noise measurements. I think it will be worth checking the manufacturer specifications for accuracy, some provide in the form of ± x dB, it is also worth checking whether the calibrator meets IEC 60942 standard or equivalent before making a purchasing decision.

One important factor in the calibration process is to ensure that you obtain a calibrator with the right adaptor. Most external microphones, especially the ones we tested, have a ¼” diameter so it is critical that the external microphone has good fit to ensure proper calibration. See image at 4:03 – 4:21 on this video https://www.youtube.com/watch?v=zk07tLj_SDs

This is great app, thank you for developing this. It is an excellent tool for informal sound surveys in the workplace. How far do you think you are from an app that can be used for compliance purposes provided you use the appropriately sensitive external microphones and calibration devices?

Hello Chris, regulatory agencies specify that a sound measurement instrument must conform to specific national (or international) standards. The sound level meter standard has 3 parts. NIOSH is making concrete steps towards establishing conformance of the app with part 3 of the standard, periodic testing, and developing a framework for others to follow. See our recent publication where NIOSH established compliance with IEC 61672 (ANSI S1.4): Part 3 – Periodic Testing, “Smartphone-based sound level measurement apps: Evaluation of compliance with international sound level meter standards.” Applied Acoustics, 139, 119-128. https://doi.org/10.1016/j.apacoust.2018.04.011. NIOSH has also conducted several field-based studies that show excellent agreement with professional sound measurement instruments, when used with external, calibrated microphones.

However, a sound measurement system must comply with a host of acoustical, electrical, and environmental requirements to fully meet standards. This is typically accomplished by sound instrument manufacturers. Although we did develop the app software, NIOSH does not manufacturer the hardware that it takes to make the system, as a whole, compliant with the standard.

Hi John, great question. We have tested some apps that support downloaded calibration files and found that such files did not improve the accuracy of the calibration when compared with using acoustical calibrators. It appears that such calibration files are based on some “nominal” correction values and are not specific to the external microphone’s sensitivity and frequency response – which would be required to establish proper calibration.

The NIOSH SLM app will automatically select a “nominal” correction value when a user plugs in an external microphone, and that value is based on our testing and evaluation of external microphones’ sensitivities. We recognize that this is not an optimal solution for establishing proper calibration and that is why we continue to recommend calibrating external microphone with acoustical calibrators similar to how an industrial hygienist or an occupational safety and health specialist (our main target audience) would normally calibrate their instruments prior to use. We believe this is the best practice until a calibration standard for smartphone apps is established.

I forgot to add a second question. Basically, does it matter. That is, does the use of a iMM-6 microphone , as is without calibration with a reference source or the use of calibration files, provide any advantage to the performance of the NIOSH app?

Chuck, Thank you for the prompt and clear response. I assume that you are also using a nominal correction value for the internal microphone. Given that is true, should one expect the use of an external microphone would have accuracy equivalent to that of the internal microphone? Or, perhaps, because there may be more uniformity among iphone microphones, I should simply stay with using the iphone microphone. Sorry for being such a nit pitcker.

Hi John, yes we use a nominal correction for internal microphones. And yes, we provide a correction for external microphones based on our testing of 10+ external microphones. The use of an external microphone does not add to the accuracy per se, we recommend it because you can calibrate it with an acoustical calibrator. Out of the box, it is not necessarily better than the internal microphone on a new and well-maintained iPhone that is not exposed to the elements. However, on an older iPhone where the internal microphone has been exposed to various environmental elements (humidity, temperature variations, dirt accumulation at opening, etc.), the external microphone will be a better option.

A highly useful app when it comes to sharing noise data through smartphone to relevant sources. Developing a noise exposure monitoring mobile app is a must need for every primary and secondary industry segment. It can significantly help to bring noise pollution under control across various parts of the worlds prevalent due to human error.

Hi George, the microphone shown in the images on our website is the MicW i437L (L stands for Lightning). It’s the same microphone that we tested in our study https://asa.scitation.org/doi/full/10.1121/1.4964639 but this one works with lightning ports. The app recognizes the external microphone automatically and assigns it a nominal sensitivity based on our laboratory testing. We made a couple of updates to the app since November 2018 to address the issue with the iPhone XS, so no additional “tuning” is required.

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